(1) Field of the Invention
The present invention relates to a powder top-blow refining lance for use in blowing a refining additive in powder form such as powder flux into molten metal such as molten steel under vacuum.
(2) Description of the Prior Art
Recently there is a growing demand for development of high-quality metallic materials. Such demand includes needs for improved mechanical properties and higher precision in the control of chemical components. A prevailing practice directed to meeting such demand is such that molten metal refined in a converter, electric furnace or any other suitable furnace is further refined under vacuum to produce a metal having the desired characteristics and composition.
For the purpose of such further refining, a refining additive in powder form is jetted through a top blowing lance onto the molten metal. Since the object of such top blow lancing is to pass the refining powder into the molten metal, it is essential to increase the flow velocity of the powder penetrating into the molten metal. Another consideration needed is to minimize possible wear of the lance interior. In order to meet these requirements there have been employed lances of single straight pipe type.
In connection with the use of lances of such conventional type various measures have been used to increase the flow velocity of the carrier gas in order to increase the flow velocity of the powder. With a lance of single straight-pipe structure, however, the difficulty is that the flow velocity of the gas, if increased, has its limit (which is Mach 1 at the most), that of the accompanying powder being inevitable lower than Mach 1. Moreover, the wear of the lance interior tends to increase proportionally as the flow velocity of the powder increases. Another difficulty is that since it is necessary to top blow from a certain or higher level above the surface of the molten metal allowing for thermal damages with the lance, the flow velocity of the powder tends to decrease appreciably before it reaches the surface of the molten metal, so that it does not allow sufficient penetration of the powder into the molten metal.
Thus, conventional single straight-pipe type lance has these disadvantages:
(1) that it has its inherent limitations which any attempt to increase the flow velocity of the powder cannot overcome;
(2) that it is liable to considerable fly loss of powder at its front end during lancing operation; and
(3) that the area over which streams of powder collide with the molten metal surface is so wide (which means that the powder is widely dispersed) that the depth of powder penetration into the molten metal is limited.
With such lance it is impracticable to allow progressing of various reactions between the powder and the molten metal to more than some limited extent.